1. Field of the Invention
This invention relates to an X-ray apparatus for use in the medical and industrial fields that includes an X-ray grid provided in an incidence surface side of an X-ray detector for removing scattered rays, the X-ray detector detecting X-rays emitted from an X-ray tube and transmitting through a subject. This invention is directed to a technique of removing a grid moiré pattern by the X-ray grid from an obtained image.
2. Description of the Related Art
Such conventional X-ray apparatus includes a top board for supporting a subject placed thereon, an X-ray tube for irradiating the subject with X-rays, an X-ray detector, such as a direct conversion type flat panel detector (FPD) for detecting X-rays transmitting through the subject M. The X-ray tube emits X-rays, and the X-ray detector detects an intensity distribution of the X-rays transmitting through the subject M. Thereby, X-ray fluoroscopy and radiography are conducted, through which fluoroscopic X-ray images are successively displayed as dynamic images on a display, such as a monitor.
Here, scattered rays occur when X-rays emitted from the X-ray tube transmit through the subject. The obtained image containing the scattered rays is to be blurred and unclear. Then the X-ray grid that removes scattered rays is arranged on an incidence surface side of the X-ray detector, thereby removing the scattered rays. The X-ray grid has an absorber (e.g., lead) for absorbing X-rays and a transparent body (e.g., aluminum or air) for transmitting X-rays, the absorbers and the transparent bodies being disposed alternately in parallel. Thus, the X-ray detector can detect only X-rays that transmit between the adjacent absorbers of the alternately disposed absorbers and transparent bodies, i.e. only X-rays that transmit along the transparent body between the absorbers. Consequently, a clear image having a higher contrast can be obtained.
On the other hand, a grid moiré pattern appears periodically in a fluoroscopic X-ray image having scattered rays removed therefrom by the X-ray grid due to differences between a sampling interval of the X-ray detector and an interval of the X-ray grid (an interval of the adjacent absorbers, or an interval of the adjacent transparent bodies). Various methods have been proposed conventionally for removing the grid moiré pattern (grid pattern). See, for example, Japanese Patent Publication No. 2005-21334A.
In Japanese Patent Publication No. 2005-21334A, applying different filters on each divided area of a radiation image can achieve suitable removal of the grid pattern from the radiation image. For example, a Gabor filter and a Wavelet filter are used for the filter.
Description will be given next of one example of the conventional methods of removing a grid moiré pattern by an X-ray grid with reference to FIGS. 1A through 1C and FIGS. 2A through 2C.
Now reference is made to FIG. 1A. FIG. 1A shows a fluoroscopic X-ray image obtained. An object to be observed is denoted by the symbol OB. A grid moiré pattern by an X-ray grid is denoted by the symbol G. Here, the grid moiré pattern appears such that lines extending vertically are arranged horizontally. An outer periphery of the fluoroscopic X-ray image is denoted by the symbol C by two-dot chain lines. Here, it is assumed that the X-ray detector detects X-rays transmitting through the subject and the X-ray grid using an image size of 2880×2880 pixels.
Firstly, one-dimensional Fast Fourier Transform (hereinafter, appropriately referred to as an “FFT”) is performed on an image containing a grid moiré pattern by an X-ray grid (hereinafter, appropriately referred to as a “source image”), shown in FIG. 1A, for each one horizontal line from an upper portion of the image. At this time, the number of data points is 2880 pixels in one line. The FFT and inverse Fast Fourier Transform (hereinafter, appropriately referred to as an “inverse FFT”) to be mentioned below are performed with powers-of-two number of data points. Thus, in order to calculate the number of data points of 2880 pixels through the FFT and the inverse FFT, powers-of-two number of data points including the number of data points of 2880 pixels are needed. Specifically, the number of data points of 211=2048 does not satisfy the 2880 data points. That is, at least the number of data points of 212=4096 is needed. One-dimensional FFT is performed for one line having the number of data points of 4096 points. Here, an example of one line to undergo FFT is denoted by the symbol L in FIG. 1A.
Next reference is made to FIG. 2A. FIG. 2A shows a frequency characteristic indicating the result of the one-dimensional FFT for one horizontal line in the source image. A peak frequency of the grid moiré pattern by the X-ray grid is detected for each line, as denoted by the symbol P in FIG. 2A, using a frequency characteristic of each line having undergone the FFT.
Next reference is made to FIG. 2B. A frequency characteristic for extracting grid moiré pattern components from the source image is prepared based on the detected peak frequency. The frequency characteristic is prepared with the number of data points of 2048 in accordance with the frequency characteristic shown in FIG. 2A.
Masking (filtering) of the frequency characteristic, shown in FIG. 2A, for each line having undergone one-dimensional FFT from the upper portion of the source image is performed with the frequency characteristic, shown in FIG. 2B, that are prepared based on the peak frequency. Specifically, calculation is performed through multiplying the frequency characteristic of FIG. 2A by that of FIG. 2B. Thereby, only frequency components of the grid moiré pattern as shown in FIG. 2C are to be extracted.
One-dimensional inverse FFT is performed to the frequency characteristic of only the grid moiré pattern components for each line having undergone masking, as shown in FIG. 2C. Thereby, a grid moiré pattern image as denoted by the symbol G′ in FIG. 1B is to be prepared. In this case, the inverse FFT is performed with the number of data points 4096 that is obtained through adding folding components to the number of data points 2048 for the frequency characteristic with the grid moiré pattern for each line having undergone masking being extracted.
Then, the grid moiré pattern image shown in FIG. 1B that is prepared through the inverse FFT is subtracted from the source image shown in FIG. 1A, whereby the grid moiré pattern is removed from the source image. FIG. 1C shows an image with the grid moiré pattern removed therefrom.
As mentioned above, when the X-ray detector has an image size of 2880 by 2880 pixels, for example, FFT and inverse FFT are performed with use of at least the number of data points of 212=4096. Thereby, the grid moiré pattern for removal from the source image is generated.
The conventional example as above has the following drawback. Specifically, where an image process, such as fluoroscopy, has to be performed in real time, an image process such as preparing and removing a grid moiré pattern is not performed in software. In other words, the same processing is mounted on hardware, such as an FPGA (Field Programmable Gate Array), and the hardware is incorporated into the apparatus. Here, the hardware enables performance at a higher speed than the software. On the other hand, where image resolution, i.e., a pixel number is large, a calculation amount required for Fast Fourier Transform becomes extremely large. Thus, there may arise a drawback that an amount of logic and computation time necessary for hardware, such as an FPGA, increases.
For instance where the FFT is performed to data of 2880 pixels for one line, calculation requires powers-of-two number of data points including the number of data points of 2880 pixels, i.e., 4096 points. Therefore, large-scale calculation is to be needed. In this case, for example, calculation of the image for one line (2880 pixels) needs more time than that for input into hardware, such as the FPGA. As a result, it becomes impossible to perform an image process in real time. Thus an image process with hardware such as the FPGA cannot be achieved.